1. Field of the Invention
This invention relates to the diaphragm device of a photographic lens.
2. Description of the Prior Art
As the diaphragm device of the conventional ordinary photographic lens, there is known a construction in which one pin of a diaphragm blade is pivotally supported on a fixed portion while the other pin is fitted in a straight groove formed in a diaphragm driving ring and a stop-down lever engaging the aperture setting lever of a camera is turnable with the diaphragm driving ring. According to this device, when the diaphragm blade is stopped down from the maximum aperture diameter toward the minimum aperture diameter, the aperture diameter is decreased by 1/.sqroot.2 times each in accordance with the stop-down of one aperture step (1 Av by APEX indication.) In a converse case, as the aperture approaches the open aperture, the aperture diameter is enlarged by .sqroot.2 times each for the variation of one aperture step. That is, the aperture diameter varies geometrically and along with it, the stroke of the stop-down lever also varies geometrically.
In the stroke characteristic of the stop-down lever shown in FIG. 1 of the accompanying drawings, this conventional photograhic lens exhibits the curve represented by broken line A, and as shown by this curve, in the conventional construction, when aperture control is effected by the stop-down lever, the stroke of the stop-down lever per aperture step becomes smaller as the aperture diameter is smaller and thus, the control accuracy thereof becomes worse.
Therefore, there is known a construction in which a cam is provided on the diaphragm driving ring so that the stroke of the stop-down lever varies arithmetically for the variation of each aperture step, as indicated by straight line B in FIG. 1. With this construction, however, the amount of variation in aperture diameter per aperture step is great particularly in the vicinity of the open aperture as already mentioned, and accordingly, the amount of displacement of the diaphragm blade is great and therefore, in the vicinity of the first step of stop-down, the efficiency of the cam is aggravated by the frictional loss of the cam surface and at the same time, the rising speed of stop-down is reduced to aggravate the follow-up characteristic of the diaphragm blade in the vicinity of the open aperture or prolong the stop-down time up to the required aperture value.
Such disadvantage may be overcome by providing cam means for controlling the amount of displacement of the stop-down lever on a member in a path through which the drive force of the stop-down lever is transmitted to the diaphragm blade (for example, forming a cam groove in the diaphragm driving ring and causing the pin of the diaphragm blade to engage the cam groove), determining a standard setting curve as shown by solid line C in FIG. 1 so that the amount of displacement of the stop-down lever for a variation in aperture step number varies geometrically in an area x in the vicinity of the first step of stop-down wherein the amount of variation in aperture diameter is particularly great and that said amount of displacement varies arithmetically in the succeeding area y, and determining the shape of the cam so that the stop-down lever operates on the basis of the standard setting curve.
Thus, the aforementioned disadvantage is eliminated, but in all of various interchangeable lenses having different open aperture diameters, to cause the stroke of the stop-down lever to operate for the aperture step number as per this standard setting curve, it is necessary to manufacture exclusive diaphragm driving rings having different cam grooves for each of the various interchangeable lenses. To avoid this, it occurs to mind that, of three types of interchangeable lenses having different open aperture diameters, namely, lens A (having an open aperture diameter .phi..sub.A), lens B (having an open aperture diameter .phi..sub.B) and lens C (having an open aperture diameter .phi..sub.C), the shape of the cam groove of the diaphragm driving ring of the lens A is formed so as to impart to the stop-down lever the operation as per the standard setting curve and that in the other interchangeable lenses B and C having smaller open aperture diameters than the A lens, a diaphragm driving ring identical to the diaphragm driving ring of the lens A is mounted in a condition in which it has been rotated until the open aperture diameters become .phi..sub.B and .phi..sub.C, respectively, whereby the diaphragm driving ring is used as a part common to the various interchangeable lenses, whereas this encounters the following inconvenience. For example, to mount said diaphragm driving ring to the lens C having an open aperture diameter .phi..sub.C smaller by 1 Av than the open aperture diameter .phi..sub.A of the lens A, it is necessary to rotate the driving ring and mount it in a condition in which it has been stopped down by 1 Av in advance through said cam groove, and to use this as the open aperture diameter of the lens C. However, due to the stop-down resulting from such rotation during the mounting, the line of the stroke of the stop-down lever of the lens C becomes a substantially straight line as indicated by alternate long and two short dashes line in FIG. 2 of the accompanying drawings which differs from the standard setting curve. When photography is effected by the use of this lens and in a case where proper exposure is to be provided with the aperture stopped down, for example, by two steps under the shutter speed priority and automatic aperture control system, the stop-down lever is stopped at the position of a.sub.1 and creates an aperture error of .DELTA.C, so that under-exposure is provided (see FIG. 2).
Likewise, when the technique of using a cam which controls the operation of the stop-down lever on the basis of said standard setting curve is intactly used with a zoom lens or a macrolens, the following inconvenience is encountered. In most of standard zoom lenses, wide angle zoom lenses or macrolenses, the F-number is maintained constant by varying the aperture diameter in response to zooming or focusing. The zoom lens shown in FIG. 3 comprises a first lens group and a second lens group and the zooming thereof is accomplished by moving these two lens groups relative to each other. A diaphragm 1 is provided in the second lens group and is displaceable with the lens. In FIG. 3, the solid lines show the telephoto position and assuming that the diaphragm 1 (aperture diameter .phi.T) at this time is in open condition, in order to maintain the F-number constant when the lens group I' and II' have been moved to the wide angle position indicated by phantom lines, it is necessary to rotate the diaphragm driving ring with the movement of the lens group and stop down the diameter of the diaphragm 1' to .phi. W.
Accordingly, if said cam means is provided in the diaphragm driving ring of such zoom lens, when the diaphragm lies at the telephoto position indicated by 1 in FIG. 3, the relation between the aperture step number and the stroke of the stop-down lever becomes as per the standard setting curve as indicated by solid line C in FIG. 4, but when the diaphragm is moved to the wide angle position indicated at 1' in FIG. 3 by zooming, the diaphragm driving ring is rotated with such movement of the diaphragm to cause the pin of the diaphragm blade to slide in the cam groove and therefore, the relation between the aperture step number an the stroke of the stop-down lever creates an error of .DELTA.d with respect to the standard setting curve, as indicated by phantom line C' in FIG. 4.
Also, there are some lenses in which only the aperture diameter is varied in response to zooming with the position of the diaphragm remaining fixed, but such lenses encounter a similar inconvenience.